SIGNALING SYSTEM 7 (SS7)

WHAT IS SIGNALING? Signaling refers to the exchange of

information between call components required to provide and maintain service.

As users of the PSTN, we exchange signaling with network elements all the time.

SS7 is a means by which elements of the telephone network exchange information.

SIGNALING SYSTEM 7 (SS7) Signaling System 7 (SS7) is an

international telecommunications standard that defines how network elements in a public switched telephone network (PSTN) exchange information over a digital signaling network.

Nodes in an SS7 network are called signaling points.

OUT-OF-BAND SIGNALING SS7 uses out-of-band signaling, which means

that signaling (control) information travels on a separate, dedicated 56 or 64 Kbps channel rather than within the same channel as the telephone call.

Historically, the signaling for a telephone call has used the same voice circuit that the telephone call traveled on (this is known as in-band signaling).

Using SS7, telephone calls can be set up more efficiently and special services such as call forwarding and wireless roaming service are easier to add and manage.

WHY OUT-OF-BAND SIGNALING? Out-of-band signaling has several

advantages that make it more desirable than traditional in-band signaling. It allows for the transport of more data at

higher speeds (56 kbps can carry data much faster than MF outpulsing).

It allows for signaling at any time in the entire duration of the call, not only at the beginning.

It enables signaling to network elements to which there is no direct trunk connection.

USED OF SS7 SS7 is used for these and other services:

Setting up and managing the connection for a call Tearing down the connection when the call is

complete Billing Managing call forwarding, calling party name and

number display, three-way calling, and other Intelligent Network (IN) services

Toll-free (800 and 888) and toll (900) calls Wireless as well as wireline call service including

mobile telephone subscriber authentication, personal communication service (PCS), and roaming

SS7 MESSAGES Information is conveyed in the form of

messages. SS7 messages can convey information such as: I’m forwarding to you a call placed from 212-555-

1234 to 718-555- 5678. Look for it on trunk 067. Someone just dialed 800-555-1212. Where do I

route the call? The called subscriber for the call on trunk 11 is busy.

Release the call and play a busy tone. The route to XXX is congested. Please don’t send

any messages to XXX unless they are of priority 2 or higher. I’m taking trunk 143 out of service for maintenance.

SIGNALING SYSTEM 7 (SS7) SS7 consists of a set of reserved or

dedicated channels known as signaling links.

There are three kinds of network points signaling pointsService Switching Points (SSPs)Signal Transfer Points (STPs)Service Control Points (SCPs)

SSP, STP, SCP signal switching points (SSPs)

SSPs are telephone switches (end offices or tandems) equipped with SS7−capable software and terminating signaling links. They generally originate, terminate, or switch calls.

signal transfer points (STPs) STPs are the packet switches of the SS7 network.

They receive and route incoming signaling messages towards the proper destination. They also perform specialized routing functions

signal control points (SCPs) SCPs are databases that provide information

necessary for advanced call-processing capabilities

SSP STP SCP

STPs and SCPs are customarily deployed in pairs. While elements of a pair are not generally co-located, they work redundantly to perform the same logical function. When drawing complex network diagrams, these pairs may be depicted as a single element for simplicity, as shown below.

BASIC SIGNALING Architecture

BASIC SIGNALING STPs W and X perform identical functions. They are

redundant. Together, they are referred to as a mated pair of STPs. Similarly, STPs Y and Z form a mated pair.

Each SSP has two links (or sets of links), one to each STP of a mated pair. All SS7 signaling to the rest of the world is sent out over these links. Because the STPs of a mated pair are redundant, messages sent over either link (to either STP) will be treated equivalently.

The STPs of a mated pair are joined by a link (or set of links).

Two mated pairs of STPs are interconnected by four links (or sets of links). These links are referred to as a quad.

SCPs are usually (though not always) deployed in pairs. Signaling architectures such as this, which provide

indirect signaling paths between network elements, are referred to as providing quasi-associated signaling.

SS7 LINKS INTERFACES

SS7 LINKS INTERFACES A link (access)

Connects signaling end point (SCP or SSP) to STP B link (bridge)

Connects an STP to another STP(STPs from a network connect to STPs of another network)

C link (cross) Connects STPs performing identical functions, forming a mated pair

(for greater reliability) D link (diagonal)

Connects a secondary (local or regional) STP pair to a primary (inter-network gateway) STP pair in a quad-link configuration; the distinction between B and D links is arbitrary

E link (extended) Connects an SSP to an alternate STP

F link(fully associated) Connects two signaling end points (SSPs and SCPs) in the same

local network

LAYERS OF THE SS7 PROTOCOL the SS7 network is an interconnected

set of network elements that is used to exchange messages in support of telecommunications functions.

The SS7 protocol isdesigned to bothfacilitate these functions and to maintain the network over which they are provided. Like most modern protocols, the SS7 protocol is layered.

LAYERS OF THE SS7 PROTOCOL Massege Transfer part 1 Massege Transfer part 2 Massege Transfer part 3 Signaling Connection Control Part Transaction CapabilitiesApplication Part Mobile Application Part IN Application part

MTP1 (PHYSICAL LAYER): MTP1 is equivalent to the physical layer

of the OSI model MTP1 defines the physical and electrical

characteristics of the signaling link. The physical interfaces defined for SS7

include E-1 (2048kb/s; 3264kb/s channels), DS-1(1544kb/s; 24 64kb/s channels), V.35(64kb/s), DS-0(64kb/s) and DS-0A(56kb/s)

MTP2 (DATA LINK LAYER): MTP2 ensures reliable end-to-end transmission

of a message across a signaling link that directly connects 2 nodes.

Implements flow control, message sequence validation, error checking and error recovery.

ISDN USER PART: ISUP ISUP defines one call control protocol

used to set-up, manage and release circuits that carry voice and data calls in the PSTN.

ISUP uses the MTP for routing messages from one SSP to another

SCCP provides connectionless and connection-oriented network services via MTP3 for the transfer of signaling messages between SSP's.

SIGNALING CONNECTION CONTROL (USER) PART: SCCP While MTP3 provides point codes to allow

messages to be addressed to specific signaling points, SCCP provides Subsystem Numbers (SSN) to let messages be addressed to specific applications at these signaling points. MTP transfers messages node-to-node while SCCP transfers messages end-to-end.

SCCP is used as the transport layer for TCAP based services like, freephone(800/888), local number portability and roaming.

TRANSACTION CAPABILITIES APPLICATION (USER) PART: TCAP TCAP messages are destined for application entities.

TCAP is designed for non-circuit related messages. TCAP provides a means for SCP-to-SCP communication

via STP's. TCAP messages are used by SSPs to query an SCP (via an

STP) to determine the routing numbers associated with a global title. The SCP uses TCAP to return a response containing the routing numbers back to the SSP via STP.

TCAP messages are transfered end-to-end using the services of SCCP.

TCAP is not limited to database access, it is also used to invoke other features from remote switches. TCAP services include freephone, calling card, and wireless roaming.

MOBILE APPLICATION PART (MAP) Mobile Application Part messages sent

between mobile switches and databases to support user authentication, equipment identification, and roaming are carried by TCAP